This invention relates to dc motors and more particularly dc motors with magnet rotors having reduced rotational noise.
FIG. 5 shows in a half-sectioned view a conventional magnet rotor type brushless dc motor having a magnet rotor, including a fixed aluminum frame 1 having a cylindrical center boss portion 2 and a stator core 3 fixed by a friction or press-on fit on the circumference of the center boss portion 2. A cup-shaped steel rotor frame 6 is fixed by a friction or press-on fit on a rotational shaft 5 journalled in the boss portion 2 through bearings 4. A rotor magnet 8 is fixed on the inner side of the rotor frame 6 in face-to-face relation with the stator core 3 through an air gap 7.
With the above-described structure, the electromagnetic attractive force between the torque generating core 3 and the magnet 8 varies with the rotational position of the magnet 8. Therefore, in order to minimize the vibrations resulting from such variations, one generally aligns the axial center planes X and X'. However, this procedure is sometimes made difficult by the presence of a Hall element 9, which detects the position of the rotor magnet 8, or by the presence of a speed sensor element 10. In such cases, rotational noise of the motor can be produced by axial vibrations due to the variable electromagnetic attractive force generated therein.